Blair Hill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Blair Hill Observatory clear sky chart from 2005-11-06 to 2026-04-17.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	11% 7% 3 7% 5% 5% 5% 4 6% 8% 37%	131843
	Night	11% 7% 3 7% 5% 4 5% 4 6% 8% 40%	130330
Transparency	Day	42% 7% 12% 21% 15% 4	135462
	Night	40% 7% 12% 23% 15% 3	134768
Seeing	Day	19% 13% 23% 22% 16% 7%	17149
	Night	17% 12% 23% 29% 17% 2	27313

By Month

Percentage Cloud Cover SE (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	14% 8% 3 7% 4 4 4 3 5% 6% 42%	5258
	11th to 20th	Day	8% 8% 5% 8% 5% 4 4 3 5% 5% 44%	5280
	21st to end	Day	13% 10% 4 7% 4 4 4 3 4 5% 43%	5780
February	1st to 10th	Day	10% 9% 3 9% 6% 5% 5% 4 5% 6% 37%	1890
	11th to 20th	Day	17% 7% 3 9% 6% 5% 5% 5% 4 7% 34%	1931
	21st to end	Day	16% 8% 4 9% 5% 4 4 4 5% 5% 36%	1785
March	1st to 10th	Day	14% 9% 3 8% 4 5% 5% 4 6% 7% 35%	5045
	11th to 20th	Day	9% 6% 3 6% 4 4 4 3 5% 7% 48%	5090
	21st to end	Day	13% 9% 3 7% 5% 5% 4 4 5% 6% 40%	5586
April	1st to 10th	Day	9% 7% 3 6% 5% 5% 4 4 5% 8% 44%	2357
	11th to 20th	Day	11% 8% 3 6% 4 4 5% 4 6% 7% 43%	2446
	21st to end	Day	14% 7% 3 7% 6% 5% 5% 4 5% 6% 39%	2570
May	1st to 10th	Day	10% 7% 3 9% 6% 6% 7% 5% 8% 10% 29%	5862
	11th to 20th	Day	17% 8% 3 8% 6% 5% 6% 5% 7% 10% 26%	5861
	21st to end	Day	12% 6% 2 7% 5% 5% 6% 5% 7% 10% 34%	6433
June	1st to 10th	Day	9% 6% 2 6% 5% 6% 6% 5% 7% 10% 39%	2600
	11th to 20th	Day	7% 6% 2 7% 6% 6% 6% 5% 8% 11% 37%	2600
	21st to end	Day	8% 6% 2 6% 5% 6% 6% 5% 8% 10% 38%	2600
July	1st to 10th	Day	6% 5% 2 7% 5% 6% 6% 5% 8% 11% 38%	5900
	11th to 20th	Day	7% 5% 3 8% 6% 6% 7% 6% 9% 13% 30%	5900
	21st to end	Day	9% 8% 3 9% 7% 7% 7% 6% 8% 10% 25%	6424
August	1st to 10th	Day	13% 7% 2 8% 6% 7% 7% 5% 8% 10% 27%	4234
	11th to 20th	Day	8% 5% 3 7% 6% 6% 7% 6% 9% 10% 33%	4174
	21st to end	Day	6% 4 2 6% 4 5% 5% 5% 7% 10% 47%	4388
September	1st to 10th	Day	10% 6% 2 7% 5% 4 4 4 6% 8% 44%	2315
	11th to 20th	Day	14% 6% 3 6% 5% 5% 5% 5% 6% 8% 38%	2200
	21st to end	Day	18% 8% 3 8% 5% 5% 5% 4 6% 6% 33%	2200
October	1st to 10th	Day	14% 6% 2 5% 3 3 4 2 5% 6% 49%	2038
	11th to 20th	Day	12% 7% 2 6% 3 4 4 4 5% 6% 48%	2000
	21st to end	Day	12% 7% 2 6% 4 4 4 3 4 6% 46%	2200
November	1st to 10th	Day	10% 5% 2 5% 4 4 4 3 6% 7% 49%	2057
	11th to 20th	Day	10% 7% 2 6% 4 4 4 4 5% 8% 45%	1901
	21st to end	Day	12% 8% 2 6% 5% 5% 3 3 5% 6% 44%	1890
December	1st to 10th	Day	9% 8% 3 7% 5% 5% 4 4 5% 6% 44%	3020
	11th to 20th	Day	15% 10% 3 6% 3 4 4 4 6% 8% 36%	3463
	21st to end	Day	14% 10% 4 8% 5% 6% 5% 5% 7% 7% 30%	4565
January	1st to 10th	Night	11% 9% 5% 9% 5% 4 4 3 6% 6% 38%	7635
	11th to 20th	Night	11% 7% 3 10% 5% 4 3 3 4 5% 44%	7680
	21st to end	Night	13% 10% 3 6% 4 3 4 3 5% 6% 43%	8401
February	1st to 10th	Night	13% 8% 3 7% 5% 4 5% 4 6% 7% 39%	2730
	11th to 20th	Night	18% 8% 3 7% 6% 4 5% 3 5% 6% 34%	2573
	21st to end	Night	12% 9% 3 8% 5% 5% 5% 4 5% 7% 37%	2019
March	1st to 10th	Night	16% 7% 3 7% 4 5% 4 4 5% 5% 40%	5028
	11th to 20th	Night	11% 7% 2 5% 3 4 4 3 5% 8% 49%	5090
	21st to end	Night	11% 9% 3 7% 5% 4 5% 3 6% 6% 41%	5588
April	1st to 10th	Night	10% 7% 3 7% 5% 4 5% 4 6% 8% 41%	2261
	11th to 20th	Night	12% 5% 2 6% 5% 3 4 4 6% 8% 46%	2036
	21st to end	Night	12% 7% 2 6% 5% 4 4 3 5% 7% 45%	1845
May	1st to 10th	Night	11% 8% 3 7% 5% 5% 5% 4 6% 9% 37%	4033
	11th to 20th	Night	15% 9% 4 8% 6% 5% 5% 4 6% 8% 30%	4032
	21st to end	Night	16% 6% 2 7% 5% 4 4 5% 6% 9% 35%	4421
June	1st to 10th	Night	7% 5% 2 5% 5% 5% 6% 5% 7% 11% 41%	1800
	11th to 20th	Night	8% 6% 2 6% 5% 4 5% 5% 7% 10% 44%	1800
	21st to end	Night	6% 4 2 5% 4 5% 5% 4 8% 11% 46%	1800
July	1st to 10th	Night	6% 5% 2 6% 5% 5% 6% 5% 8% 11% 40%	4058
	11th to 20th	Night	6% 6% 3 7% 5% 6% 6% 6% 9% 12% 35%	4057
	21st to end	Night	8% 7% 3 8% 6% 6% 7% 5% 7% 10% 32%	4427
August	1st to 10th	Night	13% 7% 3 9% 5% 5% 5% 4 7% 11% 32%	2915
	11th to 20th	Night	8% 5% 3 7% 6% 5% 6% 5% 7% 11% 38%	2875
	21st to end	Night	6% 4 1 5% 4 5% 5% 4 6% 9% 52%	3476
September	1st to 10th	Night	7% 6% 3 7% 4 4 4 4 6% 8% 47%	2080
	11th to 20th	Night	11% 6% 3 7% 5% 5% 4 3 6% 8% 42%	2200
	21st to end	Night	15% 6% 3 7% 6% 5% 5% 4 6% 6% 38%	2200
October	1st to 10th	Night	12% 7% 2 6% 4 3 4 3 4 7% 47%	2360
	11th to 20th	Night	12% 6% 3 6% 4 4 4 3 6% 6% 48%	2400
	21st to end	Night	12% 6% 2 6% 4 3 4 3 5% 7% 49%	2640
November	1st to 10th	Night	11% 7% 3 6% 4 4 4 3 5% 5% 49%	2461
	11th to 20th	Night	10% 7% 2 7% 4 4 4 3 5% 7% 47%	2714
	21st to end	Night	11% 7% 2 7% 4 4 5% 4 5% 7% 45%	2730
December	1st to 10th	Night	10% 7% 3 7% 5% 4 5% 4 6% 7% 44%	4380
	11th to 20th	Night	15% 10% 4 9% 5% 5% 5% 4 5% 7% 32%	4952
	21st to end	Night	14% 9% 4 9% 5% 5% 5% 5% 6% 7% 31%	6633

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	41% 5% 12% 19% 21% 2	5259
	11th to 20th	Day	37% 7% 5% 19% 25% 7%	5280
	21st to end	Day	45% 3 6% 24% 17% 4	5780
February	1st to 10th	Day	46% 6% 10% 20% 17% 2	1890
	11th to 20th	Day	49% 6% 8% 20% 14% 3	1940
	21st to end	Day	49% 5% 10% 16% 16% 4	1785
March	1st to 10th	Day	45% 6% 7% 20% 14% 8%	5045
	11th to 20th	Day	35% 5% 7% 23% 22% 9%	5090
	21st to end	Day	44% 6% 9% 22% 16% 4	5586
April	1st to 10th	Day	37% 7% 11% 24% 16% 5%	2357
	11th to 20th	Day	39% 6% 12% 23% 16% 4	2446
	21st to end	Day	44% 7% 11% 24% 11% 3	2570
May	1st to 10th	Day	48% 8% 14% 21% 9% 1	5897
	11th to 20th	Day	52% 7% 13% 20% 8% 1	5865
	21st to end	Day	42% 8% 14% 18% 14% 3	6447
June	1st to 10th	Day	38% 10% 17% 22% 12%	2600
	11th to 20th	Day	38% 11% 16% 19% 13% 2	2600
	21st to end	Day	38% 10% 16% 21% 13% 2	2600
July	1st to 10th	Day	39% 10% 18% 21% 9% 3	5835
	11th to 20th	Day	43% 11% 22% 19% 5%	5831
	21st to end	Day	51% 10% 14% 18% 7%	6240
August	1st to 10th	Day	49% 9% 18% 20% 5%	4221
	11th to 20th	Day	42% 8% 16% 22% 9% 1	4213
	21st to end	Day	32% 8% 17% 29% 12% 2	4388
September	1st to 10th	Day	38% 8% 12% 20% 18% 4	2315
	11th to 20th	Day	43% 7% 10% 24% 14% 2	2200
	21st to end	Day	51% 6% 11% 18% 14% 1	2200
October	1st to 10th	Day	31% 4 9% 21% 27% 7%	3311
	11th to 20th	Day	27% 4 6% 26% 20% 16%	3240
	21st to end	Day	28% 4 8% 28% 19% 13%	3538
November	1st to 10th	Day	34% 5% 10% 22% 24% 5%	2057
	11th to 20th	Day	37% 5% 9% 22% 23% 5%	1901
	21st to end	Day	40% 4 10% 22% 21% 2	1890
December	1st to 10th	Day	40% 6% 9% 20% 22% 4	3020
	11th to 20th	Day	43% 6% 12% 18% 17% 3	3460
	21st to end	Day	49% 7% 11% 20% 12% 1	4565
January	1st to 10th	Night	42% 7% 12% 20% 16% 3	7636
	11th to 20th	Night	40% 6% 5% 19% 24% 6%	7680
	21st to end	Night	42% 4 8% 24% 18% 4	8401
February	1st to 10th	Night	42% 5% 13% 22% 16% 1	2730
	11th to 20th	Night	50% 5% 8% 19% 14% 4	2575
	21st to end	Night	45% 8% 10% 22% 12% 5%	2019
March	1st to 10th	Night	42% 7% 7% 23% 15% 6%	5028
	11th to 20th	Night	31% 7% 7% 24% 23% 8%	5090
	21st to end	Night	43% 5% 10% 23% 16% 3	5588
April	1st to 10th	Night	38% 7% 14% 20% 17% 3	2261
	11th to 20th	Night	36% 5% 12% 23% 19% 5%	2036
	21st to end	Night	38% 6% 14% 27% 14% 2	1845
May	1st to 10th	Night	43% 9% 17% 21% 10% 1	4060
	11th to 20th	Night	50% 8% 14% 20% 7%	4033
	21st to end	Night	43% 8% 16% 22% 7% 3	4429
June	1st to 10th	Night	34% 11% 18% 23% 13% 1	1800
	11th to 20th	Night	34% 10% 19% 24% 12% 1	1800
	21st to end	Night	30% 12% 18% 24% 14% 2	1800
July	1st to 10th	Night	36% 12% 18% 22% 10% 2	4015
	11th to 20th	Night	39% 11% 25% 19% 5% 1	4006
	21st to end	Night	44% 11% 16% 20% 9%	4297
August	1st to 10th	Night	45% 8% 21% 20% 6%	2904
	11th to 20th	Night	37% 9% 19% 27% 9% 1	2902
	21st to end	Night	29% 7% 14% 37% 12% 1	3476
September	1st to 10th	Night	35% 9% 14% 21% 17% 4	2080
	11th to 20th	Night	40% 6% 12% 24% 17% 1	2200
	21st to end	Night	45% 7% 11% 21% 14% 2	2200
October	1st to 10th	Night	36% 3 9% 23% 25% 4	3854
	11th to 20th	Night	27% 4 9% 24% 23% 13%	3899
	21st to end	Night	24% 3 9% 30% 23% 11%	4252
November	1st to 10th	Night	38% 5% 9% 22% 22% 5%	2461
	11th to 20th	Night	37% 4 8% 22% 22% 7%	2714
	21st to end	Night	38% 5% 9% 26% 19% 4	2730
December	1st to 10th	Night	39% 4 9% 28% 19% 2	4380
	11th to 20th	Night	49% 7% 11% 17% 13% 3	4954
	21st to end	Night	46% 8% 14% 19% 12% 1	6633

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	36% 24% 24% 10% 5%	330
	11th to 20th	Day	36% 25% 27% 11% 1	330
	21st to end	Day	35% 23% 27% 11% 4	363
February	1st to 10th	Day	26% 24% 33% 12% 5%	330
	11th to 20th	Day	43% 28% 20% 8% 1	330
	21st to end	Day	34% 22% 29% 11% 3	328
March	1st to 10th	Day	25% 22% 26% 19% 6% 1	540
	11th to 20th	Day	28% 21% 26% 17% 6% 1	540
	21st to end	Day	25% 19% 32% 19% 5%	594
April	1st to 10th	Day	21% 24% 24% 21% 9% 1	540
	11th to 20th	Day	20% 18% 29% 19% 11% 3	516
	21st to end	Day	22% 14% 29% 19% 12% 4	500
May	1st to 10th	Day	18% 19% 31% 18% 11% 3	503
	11th to 20th	Day	19% 12% 28% 21% 14% 6%	527
	21st to end	Day	16% 8% 23% 26% 17% 10%	573
June	1st to 10th	Day	11% 6% 27% 27% 18% 10%	530
	11th to 20th	Day	8% 7% 22% 29% 22% 11%	530
	21st to end	Day	10% 6% 25% 27% 21% 11%	530
July	1st to 10th	Day	11% 3 15% 27% 26% 17%	525
	11th to 20th	Day	6% 3 16% 31% 27% 17%	524
	21st to end	Day	9% 3 17% 28% 29% 14%	576
August	1st to 10th	Day	17% 4 18% 23% 23% 14%	526
	11th to 20th	Day	10% 4 15% 27% 27% 18%	519
	21st to end	Day	9% 4 14% 27% 26% 21%	583
September	1st to 10th	Day	10% 5% 15% 27% 29% 15%	530
	11th to 20th	Day	14% 4 12% 27% 29% 14%	530
	21st to end	Day	22% 8% 21% 26% 19% 3	530
October	1st to 10th	Day	17% 8% 17% 27% 24% 8%	530
	11th to 20th	Day	15% 15% 22% 24% 21% 3	530
	21st to end	Day	20% 14% 26% 22% 14% 4	583
November	1st to 10th	Day	17% 13% 27% 27% 13% 3	535
	11th to 20th	Day	25% 18% 23% 23% 9% 2	341
	21st to end	Day	26% 21% 23% 22% 7% 1	330
December	1st to 10th	Day	25% 27% 29% 15% 4	330
	11th to 20th	Day	30% 17% 27% 18% 7% 1	330
	21st to end	Day	29% 18% 27% 15% 10% 1	363
January	1st to 10th	Night	24% 23% 30% 18% 3	840
	11th to 20th	Night	24% 20% 31% 23% 2	840
	21st to end	Night	21% 24% 34% 19% 2	924
February	1st to 10th	Night	21% 18% 34% 24% 2	840
	11th to 20th	Night	30% 22% 30% 17% 1	823
	21st to end	Night	25% 18% 36% 18% 2	692
March	1st to 10th	Night	19% 18% 34% 26% 3	840
	11th to 20th	Night	25% 17% 29% 23% 7%	840
	21st to end	Night	18% 16% 37% 25% 4	924
April	1st to 10th	Night	20% 19% 30% 25% 7%	840
	11th to 20th	Night	16% 15% 29% 29% 11%	816
	21st to end	Night	17% 11% 28% 34% 10% 1	645
May	1st to 10th	Night	15% 11% 27% 30% 16% 1	600
	11th to 20th	Night	16% 7% 23% 34% 19% 2	591
	21st to end	Night	14% 3 15% 31% 33% 4	650
June	1st to 10th	Night	9% 2 16% 43% 27% 3	600
	11th to 20th	Night	10% 3 15% 34% 34% 4	600
	21st to end	Night	7% 2 12% 39% 36% 4	600
July	1st to 10th	Night	10% 2 7% 36% 40% 5%	585
	11th to 20th	Night	6% 2 8% 32% 45% 7%	582
	21st to end	Night	5% 2 9% 39% 40% 5%	657
August	1st to 10th	Night	11% 4 8% 36% 36% 5%	597
	11th to 20th	Night	10% 3 8% 34% 40% 5%	594
	21st to end	Night	10% 2 10% 32% 38% 8%	809
September	1st to 10th	Night	9% 2 11% 34% 37% 6%	800
	11th to 20th	Night	14% 4 11% 32% 34% 6%	800
	21st to end	Night	19% 6% 18% 33% 24% 1	800
October	1st to 10th	Night	16% 7% 18% 29% 27% 3	800
	11th to 20th	Night	15% 10% 23% 34% 17% 1	800
	21st to end	Night	16% 11% 24% 34% 14% 1	880
November	1st to 10th	Night	17% 9% 22% 35% 15% 2	820
	11th to 20th	Night	17% 15% 25% 33% 9%	840
	21st to end	Night	18% 16% 27% 30% 9% 1	840
December	1st to 10th	Night	22% 17% 33% 23% 5%	840
	11th to 20th	Night	19% 18% 31% 27% 5%	840
	21st to end	Night	23% 16% 28% 27% 5%	924

Page generated at 2026-04-14 02:44:26. Version ac07

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